The overall goal of this research is to understand the mechanisms that underlie the vocal communication and social behavior of nonhuman primates, in order to provide a comparative framework for studies on the evolution of language and human behavior. Subjects are free-ranging baboons (Papio cyncephalus ursinus) in the Okavango Delta, Botswana. The population is one of a few in the world where background data on matrilineal kinship, dominance rank, and reproductive history are available for almost every individual, and where animals experience high rates of predation and infanticide. Three studies examine the function of vocal communication and its relation to cognition. In one, we test the hypothesis that acoustically graded signals, supplemented by context, function to communicate specific information to others. A second project examines the acoustic cues that underlie male vocal displays in competitive interactions. A third tests whether baboons recognize the "directedness" of vocalizations and attribute some level of intent to signalers. All are designed to contrast non-human primate communication and cognition with human language, thereby exploring the extent to which monkeys may be used as animal models for the study of human communicative and cognitive disorders. Three additional studies use observations, playback experiments, and data from fecal steroids. The first two examine the relation among male testosterone, corticosterone, dominance rank, parental behavior, and aggression in a population where males compete at high rates and form long-term "friendships" with females. The second focuses on females, and examines the stress hormone corticosterone in relation to seasonality, dominance rank, reproductive state, and social support in a population where predation and stress appear to be seasonal, changes in the male dominance hierarchy often lead to infanticide, and females differ widely in dominance rank and family size. All three of these studies are designed to examine the causes and consequences of aggression and stress in a close primate relative of modern humans.